Large Emission Power over 2 µW with HighQFactor Obtained from Nanocontact Magnetic-Tunnel-Junction-Based Spin Torque Oscillator

Abstract

We have developed a new type of nanocontact structure called a “sombrero-shaped” nano-contact for a spin torque oscillator (STO) based on magnetic tunnel junctions. This structure reduces the shunt current, which passes through the capping layer, and hence improves the magnetoresistance (MR) ratio to 46% compared with an MR ratio of 3% in conventional nano-contact structures. It also displays highly stable oscillation (Q=350) with high emission power over 2 µW. This is significant in the development of new spintronic devices such as nanosized microwave generators and highly sensitive nanoscale magnetic field sensors.

Spin-Torque and Spin-Hall Nano-Oscillators

“…However, the frequency of operation of these vortex oscillators is typically low. For uniform mode oscillation, the highest power achieved so far has been 2 µW, obtained from a sombrero-shaped nanocontact magnetic tunnel junction based STNO with a perpendicular anisotropy free layer [75]. For in-plane magnetized MTJ-STNOs, the highest power reported was 140 nW [74] for a frequency around 5 GHz.…”

“…The use of MTJs in conventional nanocontacts is challenging, as the topmost FM layer would act to shunt the current. Nevertheless, there has been some success in creating hybrid nanocontact MTJs, often termed "sombrero" structures, which aim to marry the low linewidths inherent in nanocontact devices with the large output powers found in MTJs [75], [76]. The materials that constitute the FM layers are diverse and can be broadly classified and distinguished by their equilibrium magnetization configurations, as described next.…”

“…Recently, significant progress has been achieved in understanding the fundamentals and in overcoming these two limitations of STNOs [75], [76], [158], [182], [188], [217], [218], [219]. As can be seen in Table I, a sombrero-shaped nanocontact geometry for MTJs has been developed [75], [76] that remedies the incompatibility of conventional nanocontact geometry with MTJs. This geometry enables the optimization of either the Q factor (narrower linewidth) [76] or the output power [75].…”

In-line spin-torque nano-oscillators in perpendicularly magnetized nanowires

“…Methods to increase STNO power output include higher magnetoresistance and larger precession angle within magnetic tunnel junction (MTJ) or nanocontact (NC) STNOs, or by turning to vortex (V) STNOs, but practical devices will nevertheless likely require synchronization of multiple STNOs to achieve useful power outputs. To date, only three NC STNOs [3,4] and four V STNOs [5] have been shown to oscillate coherently.…”

Modeling of magnetization precession in spin-torque nano-oscillators with a tilted polarizer

“…[6][7][8][9][10] However, some challenges need to be solved before STNOs can be put into practical application, which includes the limitation of output power, improving the rf oscillation quality, and increasing the frequency tunability. Various solutions have recently been proposed to overcome these problems, e.g., the utilization of perpendicular magnetic anisotropy materials as the fixed layer or free layer, [11][12][13][14][15] vortex oscillators, [16][17][18][19][20][21][22] wavy-torque spin-torque oscillators, 23 a unique device structure such as a somebrero-shape, 24,25 dual spin-polarizers, [26][27][28] and tilted-polarizer STNOs (TP-STNOs). [29][30][31][32][33][34] In the TP-STNOs device, the magnetization of the fixed layer is tilted with respect to the film plane, which can be achieved by using materials with strong titled perpendicular magneto crystalline anisotropy.…”